The invention relates to manifolds for use in, for example, Coriolis-type mass flowmeters.
In response to the need to measure the quantity of material being delivered through pipelines, numerous types of flowmeters have evolved from a variety of design principles. One of the more widely used types of flowmeters is based on volumetric flow. However, under non-ideal conditions, such as where the density of the material varies with fluid temperature, where the fluid being pumped through the pipeline is polyphase (such as a slurry), or where the fluid is non-Newtonian (such as mayonnaise or other food products), volumetric flowmeters are at best inaccurate in determining the total mass or mass flow rate of material delivered. In addition, the metered delivery of liquid components for chemical reactions, and other systems in which relative mass proportions of two or more fluids are critical, may be poorly served by volumetric flowmeters.
A mass flowmeter, on the other hand, is an instrument that provides a direct indication of the mass flow rate, as opposed to volume flow rate, of material being transferred through the pipeline. Direct measurement of mass in a moving stream requires applying a force to the stream and detecting and measuring some consequence of the resulting acceleration.
One class of mass measuring flowmeters is based on the well-known Coriolis effect. Examples of Coriolis-type mass flowmeters are described in U.S. Pat. No. 4,891,991 to Mattar et al., entitled "Coriolis-Type Mass Flowmeter," issued on Jan. 9, 1990, U.S. Pat. No. 4,911,020 to Thompson, entitled "Coriolis-Type Mass Flowmeter Circuitry," issued on Mar. 27, 1990, U.S. Pat. No. 5,048,350 to Hussain et al., entitled "Electromagnetic Driver and Sensor," issued on Sep. 17, 1991 and U.S. Pat. No. 5,054,326 to Mattar, entitled "Density Compensator for Coriolis-Type Mass Flowmeters," issued on Oct. 8, 1991, all assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Many Coriolis-type mass flowmeters induce a Coriolis force by oscillating the pipe sinusoidally about a pivot axis orthogonal to the length of the pipe. In such a mass flowmeter, Coriolis forces are exhibited in the radial movement of mass in a rotating conduit. Material flowing through the pipe becomes a radially travelling mass that, therefore, experiences an acceleration. The Coriolis reaction force experienced by the travelling mass is transferred to the pipe and is manifested as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
A major difficulty in these oscillatory systems is that the Coriolis force, and therefore the resulting deflection, is relatively small compared not only to the drive force, but also to extraneous vibrations. On the other hand, an oscillatory system can employ the inherent bending resiliency of the pipe as a hinge or pivot point for oscillation. This obviates the need for separate rotary or flexible joints, and thereby improves mechanical reliability and durability. Moreover, in an oscillatory system the resonant frequency of vibration of the pipe or conduit can be used as the drive frequency of the system, reducing the amount of energy needed to oscillate the conduit.
The susceptibility of an oscillatory system to external vibration can be lessened by delivering the flow stream through a pair of parallel conduit tube loops rigidly supported by the same flowmeter manifold block. As fluid flows through the two conduit tube loops, they are oscillated sinusoidally, generally 180.degree. out of phase with respect to each other, about pivot axes orthogonal to the length of the conduits. Because they are supported by the same rigid structure, the two parallel loops form a tuning fork assembly. The motion of the two loops tends to cancel out at the block, and the inertia of the block tends to isolate the loops from extraneous vibration. The dynamics of the parallel tube tuning fork assembly also reduce the amount of energy needed to drive the system, and help to reduce further the effects of extraneous vibration.
For a dual-tube Coriolis-type mass flowmeter to operate properly, the two tubes must be precisely balanced with respect to one another. Maintaining that balance depends in large measure on the flow rates through the tubes being equal, or nearly so. For example, if the fluid velocities in the tubes are unequal, the tube containing the higher velocity fluid will erode at a faster rate than the tube containing the lower velocity fluid, upsetting the balance of the system. Additionally, certain "coating" fluids will coat the inner surface of the high-velocity flow tube less than the inner surface of the low-velocity flow tube. This effect can also disturb the balance between the two tubes. Further, polyphase fluids may tend to separate as they flow through the tubes, the degree of separation being a function of the flow velocity. Thus, a polyphase fluid passing through the high-velocity flow tube can separate to a different degree than the same fluid passing through the low-velocity flow tube, which can also result in tube imbalance.
To ensure that the flow rates through the two parallel tubes remain equal, many dual-tube flowmeters connect the two tubes in series, such that the entirety of the measured fluid passes first through one tube, and then through the other. In a serial flow system, however, the diameters of the two tubes generally equal the diameter of the pipeline to which the flowmeter connects. This can be problematic, as typically larger-diameter tubes must also be longer, often considerably so, to provide the desired system dynamics. This makes the entire flowmeter larger and more difficult to package.
Alternatively, in a parallel flow system, a splitter divides fluid entering the flowmeter into two nearly equal flow streams, which are then provided to the two flowmeter tubes. A typical splitter for a parallel flow system can damage delicate fluids, such as milk products or blood. Moreover, splitters are subject to erosion, which can affect the ratio into which the flow streams are split, and often produce undesirable pressure drops and increased fluid flow velocities.